The first step of THz-NOMA is clustering users into groups that share communication resources, which is very challenging given the narrow width of the THz beams. Each user cluster is typically served by a single THz beam in beam-division multiple access (BDMA), as illustrated in Fig. REF  [1]}. However, only a small number of users can be served within a single NOMA group because user interference severely degrades the quality of decoded signals and causes a non-negligible delay in successive interference cancellation (SIC). Alternatively, users per beam can be further divided into sub-clusters (groups), where orthogonal group resources avoid inter-group interference (i.e., OMA between groups and NOMA within a group).
